I. THE EUKARYOTIC CELL
A. COMPOSITION AND FUNCTIONS OF EUKARYOTIC CELLULAR STRUCTURES
4. Other Internal Membrane-Bound Organelles
The overall purpose of this Learning Object is to learn the chemical makeup and the functions associated with the chloroplasts in eukaryotic cells.
LEARNING OBJECTIVES FOR THIS SECTION
The cell is the basic unit of life. Based on the organization of their cellular structures, all living cells can be divided into two groups: prokaryotic and eukaryotic (also spelled procaryotic and eucaryotic). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types.
Eukaryotic cells are generally much larger and more complex than prokaryotic. Because of their larger size, they require a variety of specialized internal membrane-bound organelles to carry out metabolism, provide energy, and transport chemicals throughout the cell.
Eukaryotic cells contain a variety of internal membrane-bound organelles that are not a part of the endomembrane system. These include mitochondria, chloroplasts, lysosomes, peroxisomes, vacuoles, and vesicles.
We will now look at chloroplasts.
Chloroplasts (see Fig. 41) are disk-shaped structures ranging from 5 to 10 micrometers in length. Like mitochondria, chloroplasts are surrounded by an inner and an outer membrane. The inner membrane encloses a fluid-filled region called the stroma that contains enzymes for the light-independent reactions of photosynthesis. Infolding of this inner membrane forms interconnected stacks of disk-like sacs called thylakoids, often arranged in stacks called grana. The thylakoid membrane, that encloses a fluid-filled thylakoid interior space, contains chlorophyll and other photosynthetic pigments as well as electron transport chains. The light-dependent reactions of photosynthesis occur in the thylakoids. The outer membrane of the chloroplast encloses the intermembrane space between the inner and outer chloroplast membranes (see Fig. 41).
The thylakoid membranes contain several pigments capable of absorbing visible light. Chlorophyll is the primary pigment of photosynthesis. Chlorophyll absorbs light in the blue and red region of the visible light spectrum and reflects green light. There are two major types of chlorophyll, chlorophyll a that initiates the light-dependent reactions of photosynthesis, and chlorophyll b, an accessory pigment that also participates in photosynthesis. The thylakoid membranes also contain other accessory pigments. Carotenoids are pigments that absorb blue and green light and reflect yellow, orange, or red. Phycocyanins absorb green and yellow light and reflect blue or purple. These accessory pigments absorb light energy and transfer it to chlorophyll.
They are found in plant cells and algae. Like Mitochondria, chloroplasts are surrounded by two membranes. The outer membrane forms the exterior of the organelle while the inner membrane folds to form a system of interconnected disclike sacs called thylakoids. The thylakoids are arranged in stacks called grana. The space enclosed by the inner chloroplast membrane is called the stroma. Chloroplasts replicate giving rise to new chloroplasts as they grow and divide. They also have their own DNA and ribosomes.
The thylakoid membranes contain the pigments chlorophyll and carotenoids, as well as enzymes and the electron transport chains used in photosynthesis (def), a process that converts light energy into the chemical bond energy of carbohydrates. Energy trapped from sunlight by chlorophyll is used to excite electrons in order to produce ATP by photophosphorylation. The light-dependent reactions that trap light energy and produce the ATP and NADPH needed for photosynthesis occur in the thylakoids. The light-independent reactions of photosynthesis use this ATP and NADPH to produce carbohydrates from carbon dioxide and water, a series of reactions that occur in the stroma of the chloroplast.
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